mailwatch, two MX servers

[Dave]

My current setup is Mailscanner on two MX servers, they forward to the
main GW server behind the firewall.  I considering installing
SMGateway, but it looks like they are fully commercial and the only
pricing I found was $900/yr. So anyhow, I'll just install mailwatch.

If I install mailwatch would that mean I can't use two MX servers?
Would users have to login into each MX server separately. We are
migrating to AD in about 6 months, does mailwatch support LDap from
AD?